Description: The overall goal of this project is to determine the physiological importance of the FSH-inhibin B feedback loop in sperm production in the non-human primate and to describe the cellular mechanisms involved in both the pituitary and testis. Five specific aims are proposed. These include (1) to test the hypothesis that testicular inhibin B secretion is the key negative feedback signal that determines, via its effect on FSH release, the number of sperm produced; (2) to identify the paracrine mechanisms within the testis whereby FSH enlarges the population of differentiated spermatogonia; (3) to clone the gene encoding inhibin alpha and the inhibin receptor in the monkey and to produce sufficient monkey inhibin B for in vitro studies; (4) to localize the inhibin receptor in the pituitary and to study its role in mediating the negative feedback control of FSH by inhibin B; (5) to examine the hypothesis that the germinal epithelium is the principal regulator of Sertoli cell inhibin B secretion and that the physiological role of FSH in this regard is largely permissive.